Parkinson’s Disease
Topic Notes
Parkinson’s Disease: Key Pharmacology & Pathophysiology
| Section | Highlights |
|---|---|
| Physiology | • Basal ganglia circuitry—direct (Go) vs indirect (No‑Go) pathways. • Dopaminergic neurons in substantia nigra pars compacta (SNc) project to striatum, modulating movement. |
| Pathophysiology | • Selective loss of SNc dopaminergic neurons → ↓striatal dopamine, ↑calcium‑permeable channels, oxidative stress, α‑synuclein aggregation. • Result: over‑inhibition of the thalamus → hypokinesia, rigidity, tremor. |
| Therapeutic Targets | • Restore dopaminergic tone (agonism, precursor, re‑uptake inhibition). • Modulate non‑dopaminergic systems (cholinergic, serotonergic, glutamatergic, adenosinergic). • Neuroprotection & disease‑modifying approaches (antioxidants, iron chelators). |
| Main Pharmacologic Classes | |
| A. Dopaminergic Therapies | • L‑DOPA (+peripheral decarboxylase blocker) – gold standard; ↓ motor complications if early & low‑dose. • Dopamine Agonists – pramipexole, ropinirole, rotigotine, aripiprazole; onset 30–90 min, longer action, lower dyskinesia risk. • COMT Inhibitors – entacapone, tolcapone (block L‑DOPA catechol-O‑methyltransferase); extend half‑life, reduce motor “off.” • MAO‑B Inhibitors – selegiline, rasagiline, safinamide; inhibit dopamine breakdown. |
| B. Adjunctive / Non‑Dopaminergic Agents | • Amantadine (NMDA antagonist) – reduces dyskinesia. • Anticholinergics – trihexyphenidyl, benztropine; useful for rigidity/tremor (age‑limited). • Disulfiram‑like – selegiline (MAO‑B) cross‑reactivity. • Glutamatergic modulators – riluzole, N‑Methyl‑D‑aspartate blockers. |
| C. Emerging / Disease‑Modifying | • GDNF/TGF‑β1 – neurotrophic factor trials (ongoing). • Stem‑cell therapies – intraparenchymal transplants. • Alpha‑synuclein antibodies – pride (anti‑synuclein) – early‑phase. |
| Common Adverse Effects | • L‑DOPA: nausea, orthostatic hypotension, dyskinesia, hallucinations. • Dopamine Agonists: impulse control disorders, nausea, somnolence, orthostatic hypotension. • COMT Inhibitors: diarrhea, black urine, hepatotoxicity (tolcapone). • MAO‑B Inhibitors: mild orthostatic hypotension, seizures (rare), GI upset. |
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Quick Reference Table
| Drug | Class | Mechanism | Key Indication | Dose (Adult) | Notes |
|---|---|---|---|---|---|
| L‑DOPA / Carbidopa | Precursor / DDC Inhibitor | Converts to dopamine in CNS; carbidopa blocks peripheral decarboxylation. | Motor symptoms (early & advanced). | 100–200 mg / 25 mg (2–5 × day). | 1–5 mg/kg/day. |
| Pramipexole | Dopamine agonist (D3> D2) | Partial agonist at D2‑like receptors. | Off‑time, tremor. | 0.125 mg‑8 mg (3 × day). | Taper for withdrawal. |
| Entacapone | COMT inhibitor | Blocks peripheral/neuronal O‑methylation of L‑DOPA. | Off‑time, motor fluctuations. | 200 mg × 3 × day. | Black urine – inform patient. |
| Selegiline | MAO‑B inhibitor | Inhibits monoamine oxidase B enzyme. | Early disease, adjunct. | 1 mg × 1 × day (low dose); 10–15 mg × 1 × day (standard). | Low dose safe with L‑DOPA. |
| Amantadine | NMDA antagonist | Reduces glutamate‑mediated excitotoxicity. | Dyskinesia, akinesia. | 100‑200 mg × 2 × day. | Avoid at high dose in renal insuff. |
| Trihexyphenidyl | Anticholinergic | Blocks muscarinic receptors. | Rigidity, tremor. | 1 mg × 3–4 × day. | Anticholinergic side‑effects ↑ in elderly. |
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Pharmacologic Decision Points
| Patient Factor | Preferred First‑Line | Why |
|---|---|---|
| Early PD (mild symptoms) | Low‑dose L‑DOPA + carbidopa ± MAO‑B inhibitor | Simple, reversible, benefits <5 yrs with low dose. |
| Young, high activity | Dopamine agonists first | Delays L‑DOPA complications. |
| Significant motor fluctuations | L‑DOPA + COMT inhibitor | Prolongs ON time. |
| Elderly with tremor/rigidity | Anticholinergic + L‑DOPA | Benefit outweighs cognitive risk. |
| Dyskinesia predominant | Amantadine or switch to dopaminergic agonist | Direct antidyskinetic effect. |
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Key References (for quick citation)
1. Kalia LV, Lang AE. Parkinson’s disease. Lancet. 2015;386:896‑911.
2. Poewe W, et al. Diagnosis and treatment of Parkinson’s disease: a review. JAMA. 2018;319:1045‑1057.
3. Gerhardt DC, et al. Emerging therapies in Parkinson’s disease: a 2023 update. Mov Disord. 2023;38:1139‑1154.
4. Monginis G. L‑DOPA: a review of its pharmacology, clinical use, and adverse effects. Clin Pharmacol Ther. 2020;107:321‑333.
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